The present invention relates to a novel process for the preparation of fluorinated aromatic compounds which makes the desired compounds accessible by halogen-fluorine exchange in a gentle manner.
Fluorinated aromatic compounds, in particular fluorobenzene and fluorobenzene derivatives, play an important role as intermediates in the preparation of crop-protection compositions (herbicides) and are the synthesis building blocks for pharmaceuticals, for example for the preparation of quinolonecarboxylic acids (Speciality Chemicals, Ed. Brian Pearson, Elsevier 1991, pages 15 to 77) and can also be used as precursors in the preparation of dyes.
Replacement of a halogen, preferably of chlorine or bromine in activated aromatic chlorine or bromine compounds, by fluorine is a favorable method of introducing fluorine substituents into an aromatic system. This reaction is generally carried out in the presence of dipolar aprotic solvents, which are used in comparatively large amounts, and alkali metal fluorides as a source of fluoride (U.S. Pat. No. 4,226,811, U.S. Pat. No. 4,069,262).
Die EP-A-0 146 924 relates to a process for the preparation of a beta-fluoropyridine compound by reacting an alpha- or gamma-halosubstituted .beta.-halopyridine with KF or CsF in a polar aprotic solvent, the beta-fluoropyridine product essentially being removed as soon as it forms and, where appropriate, the alpha- or gamma-halo-substituted .beta.-halopyridine being added. The reaction can be carried out in the presence of a phase transfer catalyst, although it can also proceed, as most of the examples show, in the absence of a phase transfer catalyst. The disadvantage of the process is that it is limited to certain fluoropyridines and requires large amounts of the polar aprotic solvent.
More recently, solvent-free processes have also become known. These solvent-free processes are carried out in the presence of phase transfer catalysts. Catalysts which have proven particularly advantageous are, for example, quaternary ammonium compounds or quaternary phosphonium compounds.
EP-A-0 635 303 describes a catalyst mixture which comprises a quaternary ammonium compound substituted by at least one alkoxypolyoxyalkyl radical, and also one or more quaternary ammonium salts and/or phosphonium salts and/or polyethers or crown ethers.
The quaternary ammonium compounds substituted by at least one alkoxypolyoxyalkyl radical, which correspond to component a) specified below, are either used alone or in the form of the catalyst mixtures described above for the preparation of fluoronitrobenzenes (EP-A-0 635 481), for the preparation of polyfluorinated nitrobenzenes (EP-A-0 635 482) or for the preparation of fluoronitrobenzenes (EP-A-0 635 486).
Although the yields which can be obtained using the process described above are relatively good, they are sometimes achieved at the expense of very long reaction times. The resulting low space-time yields are very disadvantageous for an industrial process since they make the industrial production considerably more expensive.